1. Field of the Invention
The present invention concerns a method for controlling a starting aid of a motor vehicle, in particular of a bus, in detail with the following steps: holding of the motor vehicle by means of a braking force by application of a braking pressure to a brake of the starting aid; release of the starting aid by reducing the braking pressure as a reaction to the detection of a starting operation which is initiated by the driver.
2. Description of the Related Art
A large number of methods for controlling a starting aid of a motor vehicle are known, dealing in particular, as in the present invention, with the completion or the release of the starting aid by reducing the braking pressure during a starting operation. The release of the starting aid can then take place in such a way that the motor vehicle cannot roll backwards on an uphill grade. This is a tricky problem since the grade resistance moment acting upon the motor vehicle to make it roll backwards changes with the angle of the grade and the weight of the motor vehicle due to different loading conditions. This problem can only be solved easily when the release of the starting aid, i.e. the decrease in the braking pressure, takes place slowly in such a way that in the worst case, that is to say at maximum load of the motor vehicle and at maximum grade, said vehicle will not roll backwards. The braking torque can hence decrease according to the drive torque of the drive motor.
The shortcoming of such a simple method is however that the release of the starting aid or the reduction in the braking pressure then takes place slowly accordingly when there is no danger of the vehicle rolling backwards, for instance when starting on a flat road or on a downhill grade. The driver of the motor vehicle experiences the decelerated start as a comfort loss since the vehicle “remains stuck”. Moreover, the fuel consumption is increased unnecessarily.
To reduce the braking pressure according to the grade actually prevailing during a starting operation, most known control methods set forth to determine the grade by detecting the inclination angle and to reduce the braking pressure according to the detected grade. Such a detection of the inclination angle or grade is described for instance in the publications DE 103 03 415 A1, DE 101 03 713 A1, DE 199 12 878 A1, DE 36 18 532 A1, EP 0 812 747 A2 and DE 196 30 870 A1. Different chronological sequences of the reduction in the braking torque are provided in detail according to the detected inclination angle. Document DE 36 18 532 A1 suggests moreover to provide a switch on a rotatable portion of a brake, whereas its switching mode changes depending on the conditions of the roadway, that is to say going uphill or going downhill. The system determines whether the switching mode of the switch has changed and only then generates a switch-off signal for the starting aid, once the switching mode has changed as requested. Although this method offers comparatively high safety against rolling backwards, the drive comfort is decreased in such a way that the motor vehicle so to say “remains stuck” on the road during the starting operation, namely as long as the change in the switching mode of the switch has not taken place yet. After remaining stuck, the vehicle starts jerkily due to the already comparatively high drive torque during the state change of the switch. The construction effort is moreover high and the system is error-prone due to the mechanical switch. A failure of the switch makes the vehicle undriveable, or, if the switch can be bypassed in case of malfunction, no starting aid is available any longer.
The object of the present invention, and what is needed in the art, is to provide a method for controlling a starting aid of a motor vehicle, in particular of a bus, with which said vehicle is safely prevented from rolling backwards in a permissible area, the drive comfort is increased and a starting aid can further be made available in case of malfunction. The method should be characterised by simple implementability in the motor vehicle and high operating reliability.